1. Field of the Invention
The present invention relates to a device for determining a misfiring in one cylinder of a multicylinder engine.
2. Description of the Related Art
When a misfire occurs in one of the cylinders in a multicylinder engine, the engine rotational speed drops at the power stroke in which the misfire occurs, and therefore, the time required for the crankshaft to turn by a certain crank angle during the power stroke of the misfiring cylinder becomes longer than that of other cylinders.
Therefore, for example, there is known a multicylinder engine where it is determined, for example, that the No. 1 cylinder has misfired when the period required for the crankshaft to turn by a certain crank angle during the power stroke of the No. 1 cylinder becomes longer than that of other cylinders (see Japanese Unexamined Patent Publication (Kokai) No. 62-228929).
Such a multicylinder engine is provided with a rotor which is made to rotate synchronously with the crankshaft, and which is provided with outer teeth, and an electromagnetic pickup is disposed on the engine body in the proximity of the outer teeth of the rotor, to produce an output pulse when facing an outer tooth. The time required for the crankshaft to turn by a certain crank angle is determined from the time interval of the generation of these output pulses.
The engine body, however, is usually supported by the vehicle body via engine mounts made of a resilient material such as a rubber, and thus the engine body vibrates in all directions during the operation thereof. Namely, during the operation of the engine, an engine vibration component exists along the direction of the rotation of the crankshaft, and accordingly, the relative speed of rotation of the engine body and the crankshaft includes an oscillating component generated by the engine vibration. Therefore, if the magnetic pickup is disposed on the engine body, as mentioned above, the time required for the crankshaft to turn by a certain crank angle becomes different from an actual time, when the engine vibration becomes high.
The operating speed of the engine in which the vibration of the engine is increased by resonance is determined by the spring constant (i.e., the rigidity) of the engine mounts. Usually, the rigidity of the engine mount is selected such that resonance occurs only when the engine speed is lower than the normal operating range (i.e., lower than an idling speed of the engine) to maintain the engine vibration at a relatively low level in the normal operating range. Nevertheless, the rigidity of the engine mount is gradually increased as the operation period of the engine is extended, because the material of the engine mounts tends to harden with an elapse of time. Therefore, after the engine is used for a long time, the operating speed at which the engine resonates is also gradually increased, and the vibration of the engine becomes high even in the normal operating speed range.
As mentioned above, this causes a variation of the time required for the crankshaft to turn by a certain crank angle, which is detected by the magnetic pickup disposed on the engine body, even when no misfire occurs. Therefore, it may be incorrectly determined that a misfire has occurred, even though such a misfire has not actually occurred.